1. Field of the Invention
The present invention relates to data communication networks which consist of a number of local area network (LAN) segments interconnected to form a virtual LAN environment; and more particularly to methods for managing data flow in such networks across a connectionless LAN backbone.
2. Background of the Invention
Historically, networks have been designed around the wired LAN segment as the basic technique for establishing network user groups. Standard network layer protocols define logical networks with a single layer two (data link layer) LAN segment in mind, with layer two bridging and layer three (network layer) routing functions used for moving data between LAN segments and layer three logical networks. However, with the emerging ATM LAN emulation mode and other LAN switching systems, the layer two boundaries become less controlled, giving rise to the concept of a virtual LAN. See, U.S. Pat. No. 4,823,338 to Chan et al., and an IEEE standard referred to as 802.1D. Nodes in a single layer two virtual LAN are found on different physical LAN segments but have the appearance to layer two processes (data link layer processes using medium access control MAC addresses) of residing on a single layer two LAN segment. This allows a unicast packet to propagate across the virtual LAN to any other station in the virtual LAN. Also, multi-destination packets generated on a particular LAN segment propagate throughout a number of interconnected LAN segments to ensure that all possible members of the virtual LAN receive the packet.
Within virtual LAN domains, multicast/broadcast frames are used by higher layer "discovery" or "advertisement" procedures to locate other systems or services within the virtual LAN domain. Systems send "data" to other systems using unicast MAC address which are either known in advance or learned through multicast/broadcast discovery and advertisement procedures. Systems send "multi-media data" using either unicast or multicast frames with special protocols to improve throughput or latency, as required.
Large virtual LANs create large multicast/broadcast domains; and the burden on the backbone network of transmitting all these multi-destination packets begins to impact overall system performance. More importantly, the users of the virtual LAN become burdened by a large number of multi-destination packets that must be inspected and processed, even when the packet is simply discarded. In fact, several layer three network protocols may co-exist in a single virtual LAN, resulting in much traffic which is irrelevant to many users in the virtual LAN, which must nonetheless process the traffic to discover that the network layer data unit carried in it relates to a protocol it does not use.
Commonly used network layer protocols include the internet protocol (IP) originally developed under DARPA, the interpacket exchange protocol (IPX) published by Novell, the Xerox network system (XNS) published by Xerox, the Banyan VINES protocol, the NetBIOS protocol published by IBM and Microsoft, Apple Talk published by Apple Computer, and the DECNet protocol published by Digital Equipment Corporation. Many network layer protocols create protocol specific domains based on the logical network identifiers. For example, the IP protocol establishes "subnet" domains based on the network number portion, and extensions, of the IP address of the frame. The IPX protocol creates logical networks based on the internal network number assigned to servers in the network. Apple Talk creates "zones". The NetBIOS protocol does not support multiple domains within a single LAN or emulated LAN, and can thus be considered to define a single (or "null") logical network at layer three, by default. These protocol specific logical networks defined at layer three, or higher layers, are called virtual networks, or VNETs in the present application. By the nature of virtual LANs according the prior art, the broadcast/multicast boundaries of the virtual LAN and of the VNETs are equal. Thus, as mentioned above, multicast/broadcast traffic for IPX networks will be received and processed by nodes which are members of an IP subnet, if both nodes fall in the same virtual LAN.
Prior art techniques have arisen to divide networks into several virtual LANs. U.S. Pat. No. 5,394,402 to Ross describes a virtual LAN architecture in a network which includes a backbone using a synchronous transfer mode (ATM) switching. The virtual LAN groupings act to limit the size of the multicast/broadcast domains by constraining the layer two addressing within the virtual LAN, and thus help manage the amount of multicast/broadcast packets which must be handled by a user of the network. To cross virtual LAN boundaries, internetworking devices providing layer three routing functions are required. Thus, when a change is made in a network having a number of virtual LANs, such as a new node being added, or a user moving from one LAN segment to another LAN segment in a different virtual LAN, the VNETs must be reconfigured for the new or moved node, such as by assigning a new layer three address to the node and the like. This complication has effects throughout the network, as the internetworking devices in the system need to learn the new information, and to learn that the old information in the case of a moved node, is obsolete. Further, individual users of the virtual LANs which may have cached the old layer two MAC address of the moved node, will lose track of the node, as it will not be able to send a packet across the virtual LAN boundary with the cached layer two MAC address. Also, the use of several virtual LANs within an organization, may place constraints on layer three network definition. For instance, the IPX network number used in the VNET of a first virtual LAN should not be used in the VNET of a second virtual LAN, because if a node moves from the first to the second, the moved node might erroneously access resources in the VNET of new virtual LAN with the network number of the VNET in old virtual LAN.
Thus, dividing a network into a number of virtual LANs, while having some benefits, also introduces complexity which may offset the benefits. It is desirable therefore to provide a more elegant method for managing traffic in large virtual LANs.